In the special case of the headrests, known systems for regulating their height comprise two spindles which are connected to the headrest and which are slidable in respective bores of a supporting piece or sheath fastened to the back part of the seat. Keeping the headrest in a given position, which has been chosen by the user, is classically achieved with the help of an indexing system one to keep the headrest in the set position and to modify this position by a simple voluntary action upon the headrest, in the upward or in the downward direction.
This indexing system is constituted by a succession of notches or recesses which are created on the spindles and into which are inserted elastic blocking means which are fitted together with the sheath. The notches, which usually have a cross section of a triangular profile, are as a rule unlocking by nature, that is to say that a mere vertical action on the headrest is sufficient to push back the elastic blocking means and to slide the spindles to another position where the blocking means insert themselves into another notch.
In a number of cases, particularly at the moment of impact from the rear on a vehicle equipped with these headrests, the strain exerted by the head of the user onto the headrest is liable to attain about one hundred kilograms (1000 N) with a vertical component of about 25 kg (250 N). Such a load is sufficient to unlock the indexing system and to bring about a lowering of the headrest, this being dangerous for the user whose head is no longer correctly supported and can swing backwards with the concomitant risks of injury for the cervical vertebra.
In order to avoid this problem, it has already been suggested to configure the notches of the spindles with an unsymmetrical cross section, in such a manner that the locking is positive in the downward direction, in other words it prevents a downward sliding of the spindle, even in the case of an important load, while nevertheless preserving the function of the self-unlocking during an upward sliding. In order to lower the headrest, it is then required to unlock it by acting upon the elastic blocking means, through the intermediary of an adapted pushing device, and to hold them in the unlocked position while one is lowering the headrest. This handling is thus rather impractical since it requires the use of the two hands, one for maintaining the pushing device and the other for operating the sliding of the headrest, and this operation is practically impossible, or very dangerous to be executed by the driver of a car while he is driving.